Suppose $v_0$ denotes initial value of, say, velocity, and $v_y$ denotes the $y$ component of the velocity. Is there common way of writing the $y$ component of $v_0$.
- $v_{0y}$ does not look meaningful, as it is not immediately clear that $y$ is not related to $0$ by itself. (Or that it is not some product.)
- $v_{0_y}$ does not look meaningful
- $(v_0)_y$ looks unnecessarily busy
- $v_{y}^{0}$ (or the other way round) looks confusing (because one of the subscripts became a superscript)
Velocity can be treated mathematically as an $\mathbb{R}^{n}$-valued function $v$ on the interval $[0, +\infty[$. Usually $v_{1},\dots, v_{n}$ naturally denote the component functions of $v$. So the subscripts are reserved for components. The argument of $v$ is time; so you may write $v_{1}(0), \dots, v_{n}(0)$ to denote what you want to denote.